The present invention relates to fission reactors and, more particularly, to safety mechanisms for such reactors. A major objective of the present invention is to limit corium-concrete outgas reactions upon meltdown.
Nuclear fission reactors promise to provide abundant energy with far less strain on the environment than fossil fuels. However, due to the toxicity of radioactive and other fission products, it has proved necessary to integrate several levels of back-up and safety systems into each reactor complex.
Water-cooled reactors typically include a reactor core which generates heat through fission. Recirculating water, in liquid and vapor phase, is used to transfer heat from the core to an electricity-generating turbine, or other destination. The reactor core is enclosed by a reactor vessel, which confines the recirculating fluid. The vessel itself is enclosed in a drywell, which is usually constructed with concrete walls and floor. Control rods and other reactor components extend from the vessel toward the drywell floor. The drywell is kept substantially dry to limit corrosion to these components, the vessel, and the concrete.
One of the more dramatized scenarios with which a reactor complex must be prepared to cope is a meltdown of the core when core cooling systems fail. Corium, the resulting molten material which can be at about 5000.degree. C., can breach the reactor vessel and fall to the drywell floor. The corium can react with the concrete resulting in a rapid generation of carbon dioxide and hydrogen by-products, as well as radioactive gases. The generated gas can build up pressure within the drywell, which can burst open. In this event, gaseous and particulate fission products would be released.
The corium-concrete reaction, and concomitantly its adverse affects, can be mitigated by timely flooding the drywell with water, or other coolant. Many reactor complexes include leak detection systems in the drywell which can provide early warning of a corium breach. Alteratively, temperature sensors, either within the vessel or outside the vessel and within the drywell can be used to anticipate or detect a corium breach.
The sensor data can be provided to a human operator who can then open a valve to an adjacent water reservoir. To avoid a potential for human error, feedback from a sensor can be used directly to control such a valve. However, there are still risks that the sensor or the control system or the valve itself may fail. What is needed is a system for impeding the corium-concrete reaction which is inherently less subject to failure than those discussed above.